In my post The Doctrinal Crisis on Intrinsic Evil, I discuss WHY some acts, called intrinsically evil acts, are always wrong to knowingly choose. The reason that some acts are always immoral is that God is unchanging goodness. The acts of God are not merely circumstantially good, but inherently good, because God is inherently good.

The reason that any act is intrinsically evil, is that the knowingly chosen act is ordered toward an evil end (its moral object). This intrinsic ordering of the act toward evil makes the act immoral by its very nature. But when an act is ordered toward only good in the moral object, then the act is intrinsically good — not merely good due to circumstances or intentions. Deny that acts are intrinsically evil, and you implicitly deny that acts are intrinsically good. And that is an implicit rejection of the intrinsic goodness of God.